My mother said I was a three-character idiom that was always missing things.

Madaha.

Lost three things or four things are often used to describe sloppiness or forgetfulness, either this way or that way. People who neglect things are generally very sloppy and are called Madaha.

The source of missing things is Chapter 67 of "A Dream of Red Mansions" by Cao Xueqin of the Qing Dynasty: "As the saying goes: 'The birds fly first', so as to avoid the incompleteness of missing things temporarily, which is ridiculous." Synonyms include careless and careless.